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This case has been updated. Compatible choices are kept; revisit the evidence before drawing your conclusion.
01 / The briefing
Begin with the question.
After Hubble was deployed in April 1990, its images revealed an optical problem: light was not being brought into the intended focus. The primary mirror had been shaped very precisely, but to the wrong figure. Investigators turned to the special apparatus used to measure it, a reflective null corrector. A spacing error inside that instrument had made the wrong mirror shape appear right. There were also earlier tests whose results disagreed with the principal measurement. This is a case about evidence hierarchy: when a trusted instrument and a less-favored check disagree, confidence in the trusted instrument cannot itself settle the dispute. Reconstruct how an accurate-looking test can become a systematic source of error.
The timeline
The primary mirror is completed following optical testing.
Hubble launches aboard Discovery.
Hubble is deployed.
NASA establishes an optical systems investigation board.
The board’s optical systems failure report is published.
The first servicing mission installs corrective optics and a replacement camera.
A researched reconstruction for learning. These reasoning steps simplify a complex record; no score can capture the full history.
02 / The evidence
What does the record show?
Read each exhibit before settling on an explanation. The numbered source links below every exhibit lead to the record behind it.
A characteristic blur
The observed problem was spherical aberration: rays reflected from different parts of the mirror did not converge as intended.
Source record [s1]
The test instrument
The null corrector had an internal spacing error of about 1.3 millimeters. That is not the size of the mirror’s surface error.
Source record [s1]
Contradictory checks
Other optical tests indicated an error, but their disagreement with the principal null-corrector result was not resolved before launch.
Source record [s2]
Correction without replacement
The first servicing mission corrected the optical effects using new instruments and corrective optics; it did not replace the primary mirror.
Source record [s3]
03 / Your reasoning
Follow the strongest explanation.
Choose an interpretation, then compare it with the evidence. You can revise your choice. The goal is understanding, not a perfect score.
04 / The findings
The chain behind the outcome.
Hubble’s primary mirror was not simply rough or carelessly polished. Its precise shape reflected a faulty measurement reference. The investigation linked the aberration to the reflective null corrector’s assembly and documented failures to resolve conflicting test evidence. The 1.3-millimeter figure describes spacing within the testing apparatus, not a 1.3-millimeter gouge in the mirror. That distinction matters: small errors in a measurement system can impose a different, highly consequential error on the object being manufactured. Hubble’s later correction also shows the value of diagnosis. Once the aberration was characterized, engineers could compensate optically without replacing the primary mirror. The general lesson is not to distrust sophisticated instruments. It is to make trust testable through independent checks, traceable calibration and an explicit process for investigating disagreement, especially when one instrument controls both fabrication and acceptance.
Take the method with you
- Repeatability is not the same as accuracy.
- Independent checks must be allowed to challenge the favored test.
- Name the location and meaning of a measurement before comparing its size.
You can read findings at any time. Exploring every reasoning step marks this case as complete in your notebook.
The open record
Sources & further reading
Read the original record and its scope before relying on a conclusion. Source references support the exhibit summaries; Faultory’s reasoning prompts are editorial reconstructions.
- s1NASA — Hubble’s mirror flaw
science.nasa.gov
- s2NASA — Optical Systems Failure Report
ntrs.nasa.gov
- s3NASA — The camera that saved Hubble
science.nasa.gov
Written and curated for Faultory by Yunus Emre Vurgun, with AI-assisted research and drafting. Sources and interpretations can need correction. Read our editorial standards or report an issue.